You don’t need to include jQuery here.
Use this chunk of code instead – and put it in the main plugin file since you’re modifying it anyways. That way if you disable the plugin, you won’t be loading this javascript in your theme.
function custom_scripts() {
wp_enqueue_script( 'tablesorter', plugins_url( 'jquery.tablesorter.min.js', __FILE__ ), array('jquery') );
}
add_action('init', 'custom_scripts');
Let us know how it goes.